Materials used in the high temperature sections of modern gas turbine engines and other similar machines require an optimized combination of mechanical properties and resistance to environmental degradation (oxidation and corrosion) at elevated temperatures. Superalloys, based on nickel, cobalt, or iron, often possess these desired properties, and have found widespread use in industry. The term "superalloy" is used to denote that class of refractory modified metal alloys specifically developed for high temperature service.
The primary reason for the oxidation resistance of components made from superalloys is that they form an oxide scale on the component surface at elevated temperatures; when the scale is adherent, it provides the component with long term protection from oxidation. The oxidation resistance of superalloy components can be further improved by applying an oxidation resistant coating to the component surface. See, e.g., commonly assigned U.S. Pat. Nos. 3,544,348 to Boone et al and 3,928,026 to Hecht et al. The composition and nature of oxide scales depends primarily on the composition of the alloy, and the environment in which the component operates. The important role that oxide scales play in determining high temperature properties has resulted in an extensive amount of study being devoted to their behavior. This study has revealed that several major types of oxide scales exist, which include simple as well as complex oxides/spinels based primarily on aluminum, cobalt, nickel, and chromium.
It is known that when certain ones of the rare earth elements (i.e., those elements with consecutive atomic numbers of 57 to 71, inclusive; also including yttrium, atomic number 39) are intentionally added in closely controlled amounts to some high temperature alloy compositions, the oxidation resistance of components made from such compositions is improved, because the oxide scale which forms on the component surface has greater resistance to spallation during use. See, e.g., U.S. Pat. No. 3,754,902 to Boone et al. A similar effect has also been observed with oxidation and corrosion resistant MCrAl type overlay coatings (where M is nickel, cobalt, iron, or mixtures thereof) which are often applied to the surface of components used in severe environments. Yttrium is typically the most preferred rare earth element added to MCrAl type coating alloys. A general discussion of the effects of rare earth additions on the properties of structural alloys and coating compositions is found in D. P. Whittle and J. Stringer, "Improvement in Properties: Additives in Oxidation Resistance", Philosophical Transactions of the Royal Society of London, Series A, Volume 295, 1980.
One obstacle which has, to date, limited the widespread use of rare earth modified superalloys is the high reactivity of rare earths such as yttrium with the molds and cores used in the investment casting processes. This is especially true in the directional solidification of superalloys, since the rare earths are highly reactive with silica, alumina, and zircon, materials commonly used to make investment casting molds and cores. Furthermore, the relatively slow rate at which solidification proceeds during directional solidification allows much time for the rare earth in the molten metal to react with the mold and core materials. The extent of the reaction which takes place during the casting process is difficult to predict and control, and as a result, the rare earth content in the component often varies from one casting to the next; sometimes, it even varies from one location to another in individual castings. Furthermore, the reaction product is chemically very stable, and it as well as the core are difficult to remove from the casting.
The metallurgy of structural alloys (high temperature alloys and superalloys) and coating alloys represents a sophisticated and well developed field. Much effort has been expended to optimize the composition of these alloys, including the definition of the amounts of elements which are desirably present, and the amounts of elements which are desirably absent. The latter elements are generally considered impurities, and while many elements can be completely eliminated from structural and coating alloy compositions, e.g., through the judicious selection of melt stock material, other elements cannot be entirely eliminated. One impurity which has long been recognized as being detrimental to certain properties is sulfur. Sulfur was initially identified as being detrimental to mechanical properties, and its presence in alloy compositions was limited for that reason. See, e.g., Merica et al, "The Malleability of Nickel", Transactions of the AIME, Volume 71, 1925. More recently, the presence of sulfur has also been identified as degrading oxidation resistance. See, e.g., Ikeda et al, "High Temperature Oxidation and Surface Segregation of Sulfur", Proceedings of the Third Japan Institute of Metals, Volume 24, 1983; and Funkenbusch et al "Reactive Element--Sulfur Interaction and Oxide Scale Adherence", Metallurgical Transactions A, Volume 16A, June 1985.
In view of the undesired effects of sulfur on mechanical properties and oxidation resistance, the sulfur level in high temperature alloys, superalloys, and coatings is typically limited to no more than about 100-300 parts per million by weight (ppmw). In some cases, more strict limits are imposed on the sulfur content. See, e.g., U.S. Pat. No. 3,853,540 to Schlatter et al, which states that the mechanical properties of nickel based alloys are improved by limiting the sulfur content to no more than about 20 parts per million. In U.S. Pat. No. 4,626,408 to Osozawa et al, the hot workability of Inconel Alloy 600 is improved by limiting the sulfur content to no more than about 10 parts per million. In U.S. Pat. No. 4,530,720 to Moroishi et al, the sulfur level in certain iron based alloys is limited to no more than 15 parts per million in order to optimize oxidation resistance.
Several methods for removing sulfur from molten metal exist. Many of these techniques involve contacting the molten metal with a rare earth compound, during which sulfur and the rare earth react to form a rare earth sulfide, and then removing the sulfide from the melt. See, e.g., Cremisio et al, "Sulfur--Its Effects, Removal or Modification in Vacuum Melting", Third International Symposium on Electroslag and Other Special Melting Technology, 1971; and U.S. Pat. Nos. 4,507,149 to Kay; 4,542,116 to Bertolacini et al; 4,385,937 to McGurty; 4,404,946 to Ototani. This article and each of these patents are incorporated by reference. Another technique for making components having low sulfur levels is to use high purity melt stock, and melting and solidifying the molten metal under high purity conditions.
Notwithstanding the advances which result from using materials which contain rare earth additions and/or which contain the low sulfur levels of the prior art, further improvements are needed. Such improvements would, for example, allow superalloy components to be used at higher service temperatures than they are currently used at, and therefore improve the efficiency of gas turbine engines and other types of machines.